Do 3-D movies like 'Avatar' cause headaches and sickness?

THE FACTS: The epic science-fiction film "Avatar" has broken box-office records. But it has had some unwanted side effects.

Many viewers have complained that "Avatar" and other 3-D films give them headaches, nausea, blurred vision and other symptoms of visually induced motion sickness. The problem, studies indicate, is that the films often cause unnatural eye movements.

Normally, when an object approaches a person, the eyes respond in two ways. They converge, or rotate inward to follow it (as an example, extend an arm with your index finger pointed up, then slowly pull it toward your nose). At the same time, as the object approaches, the eyes focus and maintain a clear image of it by changing the shape of the lens, a process called visual accommodation.

But a 3-D object flying off the screen causes sensory conflict. The eyes rotate inward to follow it, but they must also maintain a fixed focus on the display surface. So they converge without accommodating, an uncoupling of two natural processes that -- over the course of a long movie -- can be stressful.

There is no proven way to prevent this. But film buffs who have sat through multiple screenings of "Avatar" say one trick is to avoid looking at unfocused parts of the scenes, which sounds a lot easier than it is.